The present invention relates to a device comprising a spring formed of a stack and an element suspended via the spring, and to a method for manufacturing same from a stack consisting of a substrate and semiconductor layers.
In most piezoelectric microscanners, the micromirror is driven by actuators consisting of a piezoelectric layer enclosed by two electrodes and an underlying carrier layer, functioning like a bending beam. The piezoelectric actuators arranged at two opposite outer sides of the microscanner are driven in opposite phases such that the mirror arranged between the actuators and suspended at the actuators via portion springs vibrates in a torsion mode of a spring-and-mass system formed by the mirror, the actuators and the springs. Different layer thicknesses are necessitated for setting up such a system in order to realize the respective functions of the different components.
A small layer thickness of the actuators is usually sought so as to allow large deflections. With a constant force, the deflection of a bending beam increases with a decreasing layer thickness. However, the actuators usually are not to fall below a certain layer thickness since actuators which are too thin exhibit a very low resonant frequency and poor structural stability.
The mirror of the microscanner necessitates a certain minimum thickness so as to minimize dynamic deformations of the reflection area of the mirror which are excited by the dynamic excitation of the actuators.
The layer thickness of the torsion springs at which the mirror is suspended usually is determined very precisely in order to find a compromise between the material stress to be minimized when exerting a distortion and a high resonant frequency desired. While an increased layer thickness of the torsion springs desirably increases the resonant frequency of the spring-and-mass system, the material stress induced by the deformation of the torsion springs during operation is also rising, which may result in a premature breakdown of the microscanner.
Stiffening springs between the torsion springs and the actuators are to be sufficiently thick in order to avoid undesired deformations at these locations, induced by the actuators. The stiffening springs may comprise a greater thickness than the actuators.
Consequently, an MEMS (Micro Electro Mechanical System) structure in the form of a microscanner exhibits different regions of layer thicknesses which ideally are different from one another, wherein the layer thicknesses have to be determined at a high degree of freedom and realized with high precision. The mechanical and dynamic characteristics of the MEMS structure may be optimized by exactly determining the parameters and, thus in particular the layer thicknesses.
Conventional methods for realizing microscanners using piezoelectric actuators allow a difference in layer thickness between the actuators, the stiffening springs and the micromirror to be driven. The difference in layer thickness, however, is difficult to adjust as desired since the carrier layer of the actuators is limited in its maximum thickness by what is allowed by the coating process used, exemplarily thermal oxidation. In most cases, this is only a few micrometers. Actuators exhibiting these layer thicknesses, however, cannot achieve a high resonant frequency and stability. In addition, this method does not offer a way of implementing the stiffening springs arranged between the actuators and the torsion springs at an optimum thickness, since the manufacturing process is based only on a silicon layer which, separated by an oxide layer, is arranged on a substrate layer and the layer thickness of the silicon layer, except for the comparatively thin oxide layer, determines the maximum thickness of the stiffening springs. Due to the presence of only two material layers and the oxide layers, removing individual material layers for implementing areas of different thicknesses allows only a limited number of regions of different thicknesses.
The course of such a process is illustrated in FIG. 9.
Another method, described in FIG. 10, offers a way of providing a difference in layer thickness between the actuators and the mirror relatively easily. The actuators which are to be implemented to be thin are pre-etched in a time-controlled manner by back-side etching, wherein the mirror to be implemented to be comparatively thicker is subsequently etched further together with the pre-etched actuators. Using this method, the layer thicknesses of the actuators may be adjusted as desired, since the carrier layer is made of silicon and may thus be brought to relatively any layer thickness during an etching process. The precision of the layer thicknesses over the lateral extensions of the layers and the angle of the etching edge, however, are difficult to ensure, due to the inhomogeneous etching rate over the entire wafer. Thus, this method is suitable only for adjusting layer thicknesses in the mechanically passive regions, exemplarily the mirror or the stiffening springs. The method cannot be used for mechanically active regions, like torsion springs, the characteristics of which, in particular the natural frequency and permanent stressability, are influenced and determined by the layer thickness, due to the imprecisions mentioned before, since the layer thickness of the torsion spring which necessarily must be calculated precisely cannot be realized.
FIG. 11 shows another method for manufacturing electrostatically operated elements, exemplarily an SOI (Silicon on Insulator) wafer which consists of two silicon layers of equal layer thicknesses between which the functional layers are arranged. The method includes patterning and forming the layers from a top side of the SOI wafer towards a back side of the SOI wafer. However, this technology is not suitable for piezoelectric actuators. The functional layers for piezoelectric drives, piezoelectric layers and electrodes are ideally arranged on the topmost surface. Since it is of advantage for piezoelectric materials for the coating process to be performed on unpatterned wafers, pre-patterning from the top side where the functional layers are arranged is counterproductive.
Another example of a method for manufacturing MEMS elements comprising electrostatic drives includes a step of bonding together a patterned wafer with a second abraded wafer and subsequently coating and patterning the stack from a main side. A thickness of the wafer may be adjusted by abrading the second wafer. Since, however, piezoelectric actuators are necessitated to exhibit layer thicknesses of some micrometers, wafers would have to be abraded down to such a small thickness. Using this method, piezoelectric actuators in MEMS elements can only be manufactured at inadequate cost. In addition, adjusting the two wafers to be bonded to each other is difficult and increases the complexity of the manufacturing process.
Consequently, what would be desirable is an MEMS structure the individual components of which exhibit mutually different thicknesses and may be manufactured (cost) effectively.